Nonwoven fabric cloth substrate for printed wiring boards, and prepreg using the same

ABSTRACT

A nonwoven fabric cloth substrate for printed wiring boards containing aromatic polyamide fibers and having a 0.7-1.0 dynamic elastic modulus ratio (E&#39; (250 DEG  C.)/E&#39; (30 DEG  C.)) and a 0.05 or less loss tangent (Tan  delta ) peak value at 30-250 DEG  C., and a prepreg and a printed wiring board using the nonwoven fabric cloth substrate.

This application is a divisional of application Ser. No. 08/856,548,filed May 15, 1997, now U.S. Pat. No. 5,858,884 which application(s) areincorporated herein by reference.

FIELD OF THE INVENTION

The present invention relates to a printed wiring board useful forelectric devices. More specifically, the present invention relates to anonwoven fabric cloth substrate with superior insulating propertieswhich is useful for printed wiring boards and contains aromaticpolyamide fibers, and a prepreg and a printed wiring board comprisingthe nonwoven fabric cloth substrate.

BACKGROUND OF THE INVENTION

With the recent rapid improvement in the performance of electricdevices, more functional and economical printed wiring boards areneeded. For substrate materials, the combination of economical glassfibers having superior electric insulating properties, strength, etc.with an epoxy resin is generally used. However, the use of glass fiberscannot keep up with recent trends, such as the increase in wiringdensity and the development of chip mounting. That the use ofcopper-clad laminates (or aramid substrates) in which an aromaticpolyamide fiber nonwoven fabric cloth is applied as a substrate has beensuggested (see, for instance, U.S. Pat. No. 4,729,921, Kokai (Laid-open)Japanese Patent Application No. Sho 60-52937, No. Sho 61-160500, No. Sho62-261190, No. Sho 62-273792, No. Sho 62-274688 and No. Sho 62-274689).

Because of characteristics such as low expansion, low dielectricconstants and light weight, the application of the aramid substrates toelectric devices has been examined for public, industrial and defensepurposes.

However, the above-mentioned aramid substrate is not useful because ithas a substantial amount of impure ions and a large absorption factor.The previously known aramid substrates have a poor insulatingreliability due to the lack of adherence between the nonwoven fabriccloth and impregnated resin. The aramid substrate is also susceptable tomechanical deformation such as by warping and twisting. In order tolessen these problems, improvements in aromatic polyamide fibers, use ofa binder for the aromatic polyamide fibers, and the nonwoven fabriccloth surface quality and texture have been tested. Furthermore, thedevelopment of impregnated resins has also been tested.

SUMMARY OF THE INVENTION

In order to solve the conventional problems mentioned above, the presentinvention provides an improved nonwoven fabric cloth substrate which haslittle mechanical deformation and is useful for printed wiring boards,and a prepreg and a printed wiring board using the improved nonwovenfabric cloth substrate.

In order to achieve the objects mentioned above, the improved nonwovenfabric cloth substrate for printed wiring boards of the presentinvention comprises aromatic polyamide fibers. The ratio of dynamicelastic moduli at 250° C. and 30° C. (E' (250° C.)/E' (30° C. )) of theimproved nonwoven fabric cloth substrate is 0.7-1.0, and the peak valueof loss tangent (Tan δ) in the range of 30-250° C. is 0.05 or less. As aresult of these improved characteristics, the mechanical deformation ofthe nonwoven fabric cloth substrate is reduced, and the deformation ofthe substrate or the unevenness of the flow characteristic of animpregnated resin is prevented in the temperature range of themanufacturing process and reliability test, thereby controlling warpingand twisting.

The improved nonwoven fabric cloth is preferably treated by a heattreatment at 250-400° C. and/or a dipping treatment in an alcoholsolvent. As a result of this treatment, adherence between an impregnatedresin and the aromatic polyamide fibers in a printed wiring board (finalproduct) is improved, thereby increasing insulating reliability.

It is also preferable that the nonwoven fabric cloth is further treatedby at least one treatment selected from the group consisting of a silanecoupling agent treatment, a corona treatment and an ozone treatmentafter carrying out the heat treatment and/or the dipping treatment.

It is preferable that the aromatic polyamide fibers are para-type aramidfibers including poly(p-phenylene terephthalamide) such as "KEVLAR"(trademark) manufactured by E. I. DuPont and/or meta-aramid fibersincluding poly(m-phenylene isophthalamide) such as "NOMEX" (trademark)manufactured by E. I. DuPont.

It is also preferable that the aromatic polyamide fibers are 0.5-6.0deniers in size and have 2-14 mm fiber length.

It is preferable that the nonwoven fabric cloth substrate ismanufactured by a wet paper method. The wet papermaking method wellknown by U.S. Pat. No. 4,729,921, column 5, lines 26-51. In the method,the nonwoven cloth fabric substrate is manufactured by dispersing shortfibers together with coupling substances (e.g. aramid fibril, aramidpulp, and epoxy resin) in water, scoop them up with a net to make asheet having uniform thickness, and compress the sheet by a calendarroll.

It is also preferable that the nonwoven fabric cloth substrate has30-120 g/m² dry weight.

It is further preferable that the nonwoven fabric cloth substrate is50-300 μm thick.

The prepreg of the present invention comprises the nonwoven fabric clothsubstrate which is impregnated with a resin varnish and is then dried.

It is preferable that the resin varnish comprises an epoxy resin such astrade name "YL-6090" by YUKA SHELL EPOXY Co.

It is also preferable that the resin varnish comprises a phenol resinsuch as trade name "YLH-129", which is a novolak phenol resin, by YUKASHELL EPOXY Co.

It is further preferable that the prepreg contains the resin varnish at35-65 wt. % after being dried.

It is preferable that the prepreg is 50-200 μm thick.

It is preferable that the nonwoven fabric cloth substrate is impregnatedwith the resin varnish and is then dried after being treated with theheat treatment and/or the dipping treatment.

It is also preferable that the nonwoven fabric cloth substrate isimpregnated with the resin varnish and is then dried after the nonwovenfabric cloth is treated with the heat treatment and/or the dippingtreatment (first treatment) and after the first treatment is treatedwith the silane coupling agent treatment, the corona treatment and/orthe ozone treatment (second treatment).

The printed wiring board of the present invention comprises a prepreg inwhich the nonwoven fabric cloth substrate impregnated with a resinvarnish and then dried is applied.

By applying an effective surface treatment method which can improvebonding strength between a substrate and a resin interface, insulatingproperties improve significantly. The heat treatment and the dippingtreatment are effective to crystallize a fiber surface and removeimpurities, thus improving adherence between the substrate and theimpregnated resin as well as insulating reliability. Furthermore, thesilane coupling agent treatment, the corona treatment and the ozonetreatment (second treatment) improve the compatibility of the substratewith the impregnated resin, thus increasing adherence between thesubstrate and the impregnated resin and also insulating reliability.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an illustration explaining how to prepare samples for theadherence peeling strength measurement conducted in one embodiment ofthis invention.

FIG. 2 is an illustration explaining how to measure the adherencepeeling strength in one embodiment of this invention.

DETAILED DESCRIPTION OF THE INVENTION

The embodiments of the present invention will be described below.

EXAMPLE 1

1. Preparation of an Aromatic Polyamide Fiber Nonwoven Fabric ClothSubstrate

A nonwoven fabric cloth was prepared by a wet method from para-basedaromatic polyamide fibers ("KEVLAR" (trademark) manufactured by E. I.DuPont; size: 2.2 deniers; fiber length: 6 mm). After the nonwovenfabric cloth was compressed by using a calendar roll, a sheet which washeat-treated for ten minutes at 250° C. was used. The sheet used in thisExample was Thermount -230-240 and -244 by E.I. DuPont. The driednonwoven fabric cloth had 70 g/m² weight and 120 μm thickness after thetreatments.

2. Impregnation Process of a Resin Varnish

A varnish was prepared by mixing and dissolving the following materials:

    ______________________________________                                        Brominated bisphenol A epoxy resin                                                                    35.0   weight parts                                   (amount of bromine; 23%: epoxy equivalent; 270)                               Trifunctional epoxy resin                                                                             35.0   weight parts                                   (amount of bromine; 23%: epoxy equivalent; 270)                               Novolak phenol resin    30.0   weight parts                                   (equivalent of hydroxyl groups; 120)                                          Carbonyl diimidazole    0.1    weight part                                    Methylethylketone       66.6   weight parts.                                  ______________________________________                                    

The above mixture of the brominated bisphenol A epoxy resin and thetrifunctional epoxy resin is provided by YUKA SHELL EPOXY Co. asYL-6090, and the novolak phenol resin is also provided by YUKA SHELLEPOXY Co. as YHL-129.

Varnish was prepared by dissolving and mixing the above material. Thevanish was impregnated into the nonwoven fabric cloth substrate andheated to dry in an oven in order to remove the solvent and to proceedthe reaction of the impregnated resin, and thus, a prepreg was obtained.The time for impregnation is about from 10 seconds to two minutes.Heat-dry is conducted at about 102 to 160° C. for from three to tenminutes.

The varnish was impregnated into the nonwoven fabric cloth so as to setthe total amount of the impregnated resin at 50±1 wt. % in a prepregcondition, thereby preparing a prepreg after drying the nonwoven fabriccloth at 140° C. for five minutes. For calculating the amount of theimpregnated resin, the amount of the impregnated resin (R) is found bythe difference of the dry prepreg weight (P) after the resinimpregnation from the weight of the nonwoven fabric cloth substratebefore the impregnation. The value is then calculated as a percentage ofthe value (R) to the dry prepreg weight (P). The amount of the resincontrolled by varying conditions such as the solid concentration of theresin varnish and the impregnation time.

3. Laminating Process

After impregnating the resin varnish, electrolytic copper foils at 18 μmthickness were laminated on both surfaces of the prepreg, and werecompressed with heat by press machine treatment at 180° C. and with 50kg/cm² pressure press machine treatment for 60 minutes so as to providea copper-clad laminate.

EXAMPLE 2

A prepreg and a copper-clad laminate were manufactured as in Example 1,except that the aromatic polyamide fiber nonwoven fabric cloth describedin Example 1 was heat-treated at 300° C. for ten minutes.

EXAMPLE 3

A prepreg and a copper-clad laminate were manufactured as in Example 1,except that the aromatic polyamide fiber substrate nonwoven fabric clothdescribed in Example 1 was heat-treated at 400° C. for ten minutes.

COMPARATIVE EXAMPLE 1

A prepreg and a copper-clad laminate were manufactured as in Example 1,except that the aromatic polyamide fiber nonwoven fabric cloth describedin Example 1 was not heat-treated.

COMPARATIVE EXAMPLE 2

A prepreg and a printed wiring board were manufactured as in Example 1,except that the aromatic polyamide fiber nonwoven fabric cloth describedin Example 1 was heat-treated at 450° C. for ten minutes.

EXAMPLE 4

A prepreg and a copper-clad laminate were manufactured as in Example 1,except that the aromatic polyamide fiber nonwoven fabric cloth describedin Example 1 was dipped in methanol for about one minute.

EXAMPLE 5

A prepreg and a copper-clad laminate were manufactured as in Example 1,except that the aromatic polyamide fiber nonwoven fabric cloth describedin Example 1 was dipped in ethanol for about one minute.

EXAMPLE 6

A prepreg and a copper-clad laminate were manufactured as in Example 1,except that the aromatic polyamide fiber nonwoven fabric cloth describedin Example 1 was treated with heat at 300° C. for ten minutes and wasthen dipped in methanol for about one minute.

EXAMPLE 7

A prepreg and a copper-clad laminate were manufactured as in Example 1,except that the aromatic polyamide fiber nonwoven fabric cloth describedin Example 1 was treated with heat at 300° C. for ten minutes and wasthen treated with a silane coupling agent. Such a silane coupling agentcan be selected from the group consisting of γ-glycidoxypropyltrimethoxysilane whose trade name is A-187 by Nippon Unicar CompanyLimited, N-β-(aminoethyl)-γ-aminopropyl trimethoxysilane whose tradename is A-1120 by Nippon Unicar Company Limited, and γ-ureidopropyltriethoxysilane whose trade name is A-1160 by Nippon Unicar CompanyLimited. In this Example, γ-glycidoxypropyl trimethoxysilane was used.γ-glycidoxypropyl trimethoxysilane of concentration of 1 weight % wasdissolved in methanol, provided into a stainless steel vat, in which thenonwoven fabric cloth substrate was impregnated for one minute.

EXAMPLE 8

A prepreg and a copper-clad laminate were manufactured as in Example 1,except that the aromatic polyamide fiber nonwoven fabric cloth describedin Example 1 was treated with heat at 300° C. for ten minutes and then acorona treatment was carried out on the nonwoven fabric cloth at100W·min./m² by using a corona discharge treatment device manufacturedby KASUGA ELECTRIC WORKS LTD.

EXAMPLE 9

A prepreg and a copper-clad laminate were manufactured as in Example 1,except that the aromatic polyamide fiber nonwoven fabric cloth describedin Example 1 was treated with heat at 300° C. for ten minutes and thenan ozone treatment were carried out on the nonwoven fabric cloth at 100Wfor five minutes by using a QOL-25SY device manufactured by AI GRAPHICS.

EXAMPLE 10

A prepreg and a copper-clad laminate were manufactured as in Example 1,except that the aromatic polyamide fiber nonwoven fabric cloth describedin Example 1 was dipped in methanol and then treated with a silanecoupling agent.

EXAMPLE 11

A prepreg and a copper-clad laminate were manufactured as in Example 1,except that the aromatic polyamide fiber nonwoven fabric cloth describedin Example 1 was dipped in methanol and a corona treatment was thencarried out on the cloth.

EXAMPLE 12

A prepreg and a copper-clad laminate were then manufactured as inExample 1, except that the aromatic polyamide fiber nonwoven fabriccloth described in Example 1 was dipped in methanol and an ozonetreatment was then carried out on the cloth.

In an alternative embodiment, the treatments may be carried out duringthe process of preparing a nonwoven fabric cloth substrate as describedin Example 1. In this case, the first and second treatments are notcarried out in the process of manufacturing a prepreg.

Adherence strength between the aramid fiber nonwoven fabric clothsubstrate and the matrix resin (bonding strength between layers) as wellas the conductivities of water extract of the copper-clad laminates ofExamples 1-12 and Comparative Examples 1-2 are shown in the followingTables 1 and 2. The adherence strength has a direct influence oninsulating reliability while the conductivity is an indicator of theamount of water soluble impurities.

                  TABLE 1                                                         ______________________________________                                                    (1)  (2)        (3)    (4)                                        ______________________________________                                        Example 1     250    None       0.65 80                                       Example 2     300    None       0.75 60                                       Example 3     400    None       0.75 50                                       Comparative Ex. 1                                                                           None   None       0.50 110                                      Comparative Ex. 2                                                                           450    None       0.45 120                                      Example 4     None   Methanol   0.60 100                                      Example 5     None   Ethanol    0.60 100                                      Example 6     300    Methanol   0.85 55                                       ______________________________________                                         (1) Heat treatment temperature (° C.)                                  (2) Dipping treatment                                                         (3) Adherence strength (kg/cm)                                                (4) Conductivity (μs/cm)                                              

                  TABLE 2                                                         ______________________________________                                                 (1)  (2)        (5)        (3)                                       ______________________________________                                        Example 7  300    None       Silane coupling                                                                        0.85                                    Example 8  300    None       Corona   0.80                                    Example 9  300    None       Ozone    0.80                                    Example 10 None   Methanol   Silane coupling                                                                        0.70                                    Example 11 None   Methanol   Corona   0.70                                    Example 12 None   Methanol   Ozone    0.70                                    ______________________________________                                         (1) Heat treatment temperature (° C.)                                  (2) Dipping treatment                                                         (3) Adherence strength (kg/cm)                                                (5) Additional treatment                                                 

The following is analysis methods relating to the Tables 1 and 2.

Adherence peeling strength (kg/cm): As shown in FIG. 1, the heat-pressedcopper-clad laminate 1 was cut along the broken lines 2 to be 1 cm wideand 10 cm long, so that a sample 3 was prepared. The sample 3 was cut byusing a cutter in the middle of the layers to 2 cm from one end of thesample 3, so that holding portions (6, 6') were made. Next, as shown inFIG. 2, a peel test was carried out by using a tensile tester: theholding portions (6, 6') were held and stretched in the (F, F)directions. In FIG. 2, 4 is a substrate layer comprising an aramid fibernonwoven fabric cloth substrate and a matrix resin, while 5 and 5' arecopper layers. A scanning electron microscope (SEM) was used to measureadherence strength by detecting whether or not the aramid fiber nonwovenfabric cloth substrate was peeled off from the matrix resin.

Conductivity (μS/cm): a hardened substrate was prepared by removingcopper foils from a thermally pressed copper-clad laminate, and thesubstrate was pulverized into a powder of less than 60 mesh. Five gramsof the powder and 50 ml of pure water were added into a 100 ml sealingcontainer made of polytetrafluoroethylene ("Teflon" (trademark)manufactured by Dupont), and were treated at 121° C. for twenty fourhours. The conductivity of the solution was then measured by aconductivity meter.

Table 1 shows the effects of the heat treatment and the dippingtreatment. The adherence strength was increased in samples that wereheat treated. The preferred treatment temperature was can from 250° C.to 400° C. When the temperature was 450° C., adherence strengthdeclined. The heat treatment can also effectively decreaseconductivities. Since heat likely promotes fiber crystallization, thisprevents the elution of impurities from the fibers. In other words, thischaracteristic is effective so as to prevent poor insulation inpreparing printed wiring boards. Like the heat treatment, the dippingtreatment with methanol or ethanol also improved adherence strength. Thedipping treatment may be carried out along with the heat treatment. Thedipping treatment can clean the surface of fibers, thus increasingadherence strength. Other solvents, such as ethanol, acetone andmethylethylketone, may also be used for the dipping treatment.

Table 2 shows adherence strength when a second treatment is carried outin addition to the heat or the dipping first treatment. The silanecoupling treatment, corona treatment and ozone treatment increaseadherence strength. These treatments are generally applied so as toimprove adherence strength, and are highly effective.

EXAMPLE 13

1. Preparation of an Aromatic Polyamide Fiber Nonwoven Fabric ClothSubstrate

A nonwoven fabric cloth was prepared by a wet method from para-typearomatic polyamide fibers (96 wt. %; "KEVLAR" (trademark) manufacturedby Dupont; size: 2.2 deniers; fiber length: 6 mm) and meta-type aromaticpolyamide fibers (4 wt. %; "NOMEX" (trademark) manufactured by Dupont;size 2.2 deniers; fiber length: 6 mm). After carrying out a papertreatment, a calender treatment was carried out on the nonwoven fabriccloth at 300° C. with 200 kg/ pressure. The dry weight of the nonwovenfabric cloth was 70 g/m² and the thickness was 100 μm.

2. Impregnation Process of a Resin Varnish

A varnish was prepared by mixing and dissolving the following materials:

    ______________________________________                                        Brominated bisphenol A epoxy resin                                                                    35.0   weight parts                                   (amount of bromine: 23%; epoxy equivalent: 270)                               Trifunctional epoxy resin                                                                             35.0   weight parts                                   (amount of bromine: 23%; epoxy equivalent: 270)                               Novolak phenol resin    30.0   weight parts                                   (equivalent of hydroxyl groups)                                               Carbonyl diimidazole    0.1    weight part                                    Methylethylketone       66.6   weight parts.                                  ______________________________________                                    

The varnish was impregnated into the nonwoven fabric cloth so as to setthe total amount of impregnated resin at 50±1 wt. % in a prepregcondition. A prepreg was prepared by drying the nonwoven fabric cloth at140° C. for five minutes.

3. Laminating Process

After being dipped in the resin varnish, the prepreg was laminated withelectrolytic copper foils at 18 μm thickness on both surfaces, and wasthen thermally compressed at 180° C. and with 50 kg/cm² for 60 minutesso as to prepare a copper-clad laminate.

EXAMPLE 14

A prepreg and a copper-clad laminate were manufactured as in Example 13,except that a water-dispersed epoxy resin binder was added to thearomatic polyamide fibers at 1 wt. %. The trade name of the binder isDickfine EN-0270 manufactured by DAINIPPON INK & CHEMICALS, INC.

EXAMPLE 15

A prepreg and a copper-clad laminate were manufactured as in Example 13,except that a water-diespersed epoxy resin binder described in Example14 was added to the aromatic polyamide fibers at 3 wt. %.

COMPARATIVE EXAMPLE 3

A prepreg and a copper-clad laminate were manufactured as in Example 13,except that a water-dispersed epoxy resin binder described in Example 14was added to the aromatic polyamide fibers at 5 wt. %.

COMPARATIVE EXAMPLE 4

A prepreg and a copper-clad laminate were manufactured as in Example 13,except that a water-dispersed epoxy resin binder described in Example 14was added to the aromatic polyamide fibers at 7 wt. %.

EXAMPLE 16

A prepreg and a copper-clad laminate were manufactured as in Example 13,except that the nonwoven fabric cloth prepared as in Example 13 wastreated with heat at 250° C. for ten minutes after the calendertreatment.

EXAMPLE 17

A prepreg and a copper-clad laminate were manufactured as in Example 13,except that the nonwoven fabric cloth prepared as in Example 13 wastreated with heat at 300° C. for ten minutes after the calendertreatment.

EXAMPLE 18

A prepreg and a copper-clad laminate were manufactured as in Example 13,except that the nonwoven fabric cloth prepared as in Example 13 wastreated with heat at 400° C. for ten minutes after the calendertreatment.

COMPARATIVE EXAMPLE 5

A prepreg and a copper-clad laminate were manufactured as in Example 1,except that the nonwoven fabric cloth prepared as in Example 13 wastreated with heat at 450° C. for ten minutes after the calendertreatment.

EXAMPLE 19

A prepreg and a copper-clad laminate were manufactured as in Example 13,except that the nonwoven fabric cloth prepared as in Example 13 wasdipped in methanol after the calender treatment.

EXAMPLE 20

A prepreg and a copper-clad laminate were manufactured as in Example 13,except that the nonwoven fabric cloth prepared as in Example 13 wasdipped in ethanol after the calender treatment.

EXAMPLE 21

A prepreg and a copper-clad laminate were manufactured as in Example 13,except that the nonwoven fabric cloth prepared as in Example 13 wastreated with heat at 300° C. for ten minutes and was then dipped inmethanol after the calender treatment.

EXAMPLE 22

A prepreg and a copper-clad laminate were manufactured as in Example 13,except that the nonwoven fabric cloth prepared as in Example 13 wastreated with heat at 300° C. for ten minutes and was then treated with asilane coupling agent after the calender treatment.

EXAMPLE 23

A prepreg and a copper-clad laminate were manufactured as in Example 13,except that the nonwoven fabric cloth prepared as in Example 13 wastreated with heat at 300° C. for ten minutes and a corona treatment wasthen carried out on the cloth after the calender treatment.

EXAMPLE 24

A prepreg and a copper-clad laminate were manufactured as in Example 13,except that the nonwoven fabric cloth prepared as in Example 13 wastreated with heat at 300° C. for ten minutes and was then treated withozone after the calender treatment.

EXAMPLE 25

A prepreg and a copper-clad laminate were manufactured as in Example 13,except that the nonwoven fabric cloth prepared as in Example 13 wasdipped in methanol and was then treated with a silane coupling agentafter the calender treatment,.

EXAMPLE 26

A prepreg and a copper-clad laminate were manufactured as in Example 13,except that the nonwoven fabric cloth prepared as in Example 13 wasdipped in methanol and a corona treatment was then carried out on thecloth after the calender treatment.

EXAMPLE 27

A prepreg and a copper-clad laminate were manufactured as in Example 13,except that the nonwoven fabric cloth prepared as in Example 13 wasdipped in methanol and was then treated with ozone after the calendertreatment.

In an alternative embodiment, the first and second treatments may becarried out when preparing the nonwoven fabric cloth as described inExample 1. When the treatments are carried out in the preparationprocess of the nonwoven fabric cloth, they are not needed in the processof manufacturing a prepreg.

The following Tables 3-5 show dynamic viscoelasticity (dynamic elasticmodulus ratio and loss tangent), degree of warp, adherence strength(bonding strength between layers) and water extract conductivity of thecopper-clad laminates of the examples and the comparative examplesmentioned above.

                  TABLE 3                                                         ______________________________________                                                   Dynamic                                                                       Viscoelasticity                                                             E1/E2    Tan δ                                                                          A       B    C                                       ______________________________________                                        Example 13 1.00       0.02   1.0   0.75 100                                   Example 14 0.90       0.03   1.5   0.75 105                                   Example 15 0.70       0.05   2.0   0.80 110                                   Comparative Ex. 3                                                                        0.55       0.07   4.0   0.85 120                                   Comparative Ex. 4                                                                        0.40       0.10   6.0   0.85 130                                   ______________________________________                                         A Degree of warp (mm)                                                         B Adherence strength (kg/cm)                                                  C Conductivity (μm/cm)                                                

                  TABLE 4                                                         ______________________________________                                                  (1)   (2)       A      B     C                                      ______________________________________                                        Example 16  250     None      1.0  0.85  100                                  Example 17  300     None      1.0  0.90  75                                   Example 18  400     None      1.0  0.90  65                                   Comparative Ex. 5                                                                         450     None      3.0  0.60  120                                  Example 19  None    Methanol  1.0  0.85  100                                  Example 20  None    Ethanol   1.0  0.85  100                                  Example 21  300     Methanol  1.0  0.95  70                                   ______________________________________                                         (1) Heat treatment temperature (° C.)                                  (2) Dipping treatment                                                         A Degree of warp (mm)                                                         B Adherence strength (kg/cm)                                                  C Conductivity (μm/cm)                                                

                  TABLE 5                                                         ______________________________________                                               (1)   (2)       (5)         A    B                                     ______________________________________                                        Example 22                                                                             300     None      Silane coupling                                                                         1.0  1.05                                Example 23                                                                             300     None      Corona    1.0  1.00                                Example 24                                                                             300     None      Ozone     1.0  1.00                                Example 25                                                                             None    Methanol  Silane coupling                                                                         1.0  1.05                                Example 26                                                                             None    Methanol  Corona    1.0  1.00                                Example 27                                                                             None    Methanol  Ozone     1.0  1.00                                ______________________________________                                         (1) Heat treatment temperature (° C.)                                  (2) Dipping treatment                                                         (5) Additional treatment                                                      A Degree of warp (mm)                                                         B Adherence strength (kg/cm)                                             

Analysis methods are as follows.

Dynamic elastic modulus ratio (E' 250° C./E' 30° C.): a hardenedsubstrate was prepared after removing copper foils from the surfaces ofa thermally pressed copper-clad laminate, and the substrate was then cutto 3 mm width. The dynamic elastic modulus ratio (E') in the range of20-300° C. of the substrate was measured by a dynamic viscoelasticitymeasuring device (11 Hz and 3° C./min), so that a ratio of modulus at30° C. and 250° C. was measured. In Table 1, E' 250° C./E' 30° C. isreferred to as E1/E2.

Loss tangent (Tan δ) peak value: a hardened substrate was prepared afterremoving copper foils from the surfaces of a thermally pressedcopper-clad laminate, and the substrate was then cut to 3 mm width. Thedynamic elastic modulus ratio (E') and the loss modulus ratio (E") inthe range of 20-300° C. of the substrate were measured by a dynamicviscoelasticity measuring device (11 Hz and 3° C./min), so that amodulus ratio (Tan δ=E"/E') was measured. The modulus ratio is used as apeak value.

Degree of warp (mm): a hardened substrate was prepared after removingcopper foils from the surfaces of a thermally pressed 20 cm×20 cm sizecopper-clad laminate, and the substrate was then placed on a board. Theelevation at the four corners of the hardened substrate was measured asthe degree of warp.

Adherence strength (kg/cm): a thermally pressed copper-clad laminate wascut to 1 cm width, and a peel test was carried out ninety times by atensile tester. A scanning electron microscope (SEM) was used to measureadherence strength by examining whether or not the nonwoven fabric clothpeeled off from the matrix resin.

Conductivity (μS/cm): a hardened substrate was prepared after removingcopper foils from the surfaces of a thermally pressed copper-cladlaminate, and the substrate was pulverized into powder of less than 60mesh. Five grams of the powder and 50 ml of pure water were added into a100 ml sealed container made of polytetrafluoroethylene ("Teflon"manufactured by Dupont), and were treated at 121° C. for twenty fourhours. The conductivity of this solution was measured by a conductivitymeter.

The patterns on the copper foils of a copper-clad laminate were formedby etching, thus preparing a printed wiring board. The warp and twist ofthe printed wiring board are influenced mostly by the uneven quality ofa substrate, but are also influenced by copper patterns. The degree ofwarp was measured by using a substrate where copper foils were removedby etching, so that the measurement was not influenced by copperpatterns.

Table 3 shows a clear correlation between E1/E2, Tan δ and the degreesof warp. As E1/E2 becomes small, the degree of warp increases. WhenE1/E2 is large and close to 1.0 and Tan δ is closer to zero, a printedwiring board is thermally stable and does not change its size. The tablealso indicates that there is no uneven mechanical change in the processof manufacturing a prepreg and a copper-clad laminate. In order to keepthe degree of warp 2.0 mm or less, it is preferable that E1/E2 is 0.7 orhigher and Tan δ is 0.05 or less. As shown in Comparative Examples 3 and4, conductivity increases as the amount of a water-based epoxy resinbinder increases.

Table 4 shows the effects of the heat and dipping treatments. There isno difference in the degree of warp between a substrate treated withheat and one without the treatment, but the adherence strength improveswhen the heat treatment is carried out The temperature of the heattreatment is preferably from 250° C. to 400° C. However, adherencestrength decreases at 450° C. The heat treatment can also reduceconductivity. The crystallization of fibers by heat can prevent theelution of impurities. Like the heat treatment, the dipping treatmentwith methanol or ethanol is also effective in increasing the adherencestrength. The heat treatment can be carried out along with the dippingtreatment. The dipping treatment can clean the surface of fibers, andincrease adherence strength. The dipping treatment is also effectivewith other solvents, such as ethanol, acetone and methylethylketone.

Table 5 shows the adherence strength when another, second treatment iscarried out in addition to the heat treatment or the dipping treatment(first treatment). The adherence strength is enhanced by the silanecoupling treatment, corona treatment or ozone treatment. These secondtreatments cannot increase the adherence strength by themselves, but canbe effective when fiber surfaces are cleaned by the first heat treatmentor dipping treatment.

The invention may be embodied in other forms without departing from thespirit or essential characteristics thereof. The embodiments disclosedin this application are to be considered in all respects as illustrativeand not limitative, the scope of the invention is indicated by theappended claims rather than by the foregoing description, and allchanges which come within the meaning and range of equivalency of theclaims are intended to be embraced therein.

What is claimed is:
 1. A nonwoven fabric cloth substrate for printedwiring boards comprising aromatic polyamide fibers, wherein saidnonwoven fabric cloth substrate has a 0.7-1.0 ratio of dynamic elasticmoduli between 250° C. and 30° C. (E' (250° C.)/E' (30° C.)) and has a0.05 or less peak value of loss tangent (Tan δ) in a range of 30-250° C.2. The nonwoven fabric cloth substrate for printed wiring boards ofclaim 1, wherein a nonwoven fabric cloth is treated by at least onefirst treatment selected from the group consisting of a heat treatmentat 250-400° C. and a dipping treatment in an alcohol solvent.
 3. Thenonwoven fabric cloth substrate for printed wiring boards of claim 2,wherein the nonwoven fabric cloth is further treated by at least onesecond treatment selected from the group consisting of a silane couplingagent treatment, a corona treatment and an ozone treatment after saidfirst treatment.
 4. The nonwoven fabric cloth substrate for printedwiring boards of claim 1, wherein the aromatic polyamide fibers compriseat least one fiber type selected from the group consisting of para-typearamid fibers and meta-type aramid fibers.
 5. The nonwoven fabric clothsubstrate for printed wiring boards of claim 1, wherein the aromaticpolyamide fibers are 0.5-0.6 deniers in size and have 2-15 mm fiberlength.
 6. The nonwoven fabric cloth substrate for printed wiring boardsof claim 1, wherein said nonwoven fabric cloth substrate is manufacturedby a wet paper method.
 7. The nonwoven fabric cloth substrate forprinted wiring boards of claim 1, wherein said nonwoven fabric clothsubstrate has a 30-120 g/m² dry weight.
 8. The nonwoven fabric clothsubstrate for printed wiring boards of claim 1 wherein said nonwovenfabric cloth substrate has a 50-300 μm thickness.